Currently, it is well known that in order to accommodate sharply increasing Internet traffic, opticalization of networks with WDM communication as a core is developing rapidly. Current WDM networks are mainly in a point-to-point form. However, it is assumed that these will be developed to ring networks or mesh networks in the near future. At respective nodes constituting such a network, processing such as branching and insertion of an optical signal of an arbitrary wavelength, and optical cross connect (OXC) without involving conversion to electricity is possible, and it is considered that dynamic setting and release of the optical path are performed based on wavelength information.
FIG. 13 is a perspective view illustrating a configuration example of such a conventional wavelength selecting switch arrangeable at nodes of the network. Moreover FIG. 14 and FIG. 15 are a top view and a side view of the wavelength selecting switch in FIG. 13. This conventional wavelength selecting switch includes an input/output optical system 110, a spectral element 120, a light collecting optical system 130, a mirror section 140, and a control section (CONT) 150.
In the input/output optical system 110, a plurality of optical fibers 111 and micro lenses 112 arranged in the vicinity of one end of the respective optical fibers 111, are arranged in a single direction, to constitute one input port Pin and a plurality of output ports Pout. The WDM light provided to the input port Pin is emitted from the one end of the optical fiber 111 and is made into parallel light by the micro lens 112, and is then separated to different angular directions corresponding to the wavelength by the spectral element 120, and the lights of the respective wavelengths are collected at different positions by the light collecting optical system 130. The mirror section 140 having a plurality of reflecting mirrors 141 corresponding to the number of wavelengths is arranged at a collection position of the lights of the respective wavelengths. The respective reflecting mirrors 141 are formed by using, for example, micromachining (Micro Electro Mechanical Systems: MEMS) technology, and are minute mirrors in which an angle of a reflecting surface can be controlled corresponding to a drive signal.
The lights of respective wavelengths that have reached the mirror section 140 are respectively reflected by the corresponding reflecting mirror 141, and are folded back to a direction corresponding to the angle of the respective reflecting surfaces. At this time, the reflecting surfaces of the respective reflecting mirrors 141 are controlled by the control section 150 so as to become a predetermined angle corresponding to the position of any one output port Pout set as an output destination of the input light. As a result the lights of the respective wavelengths folded back by the respective reflecting mirrors 141 sequentially pass through the light collecting optical system 130 and the spectral element 120, and are guided respectively to the target output port Pout. Here the direction in which the light of each wavelength is angularly dispersed by the spectral element 20 is the X direction, the direction in which the input and output ports are arranged is the Y direction, and the direction of the optical axis perpendicular to the X-Y plane is the Z direction.
Such a conventional wavelength selecting switch has a wavelength selecting function for the lights of a plurality of wavelengths included in the input WDM light, that can select the light of an arbitrary wavelength and guide the light to the target output port Pout, by controlling the angle of the reflecting surface of each reflecting mirror 141. Moreover, by inverting the relation of input and output as illustrated in a side view of FIG. 16, it is also possible to select the light provided to a plurality of input ports Pin, according to the wavelength, and guide this to one output port Pout.
Furthermore the conventional wavelength selecting switch also has a function as a variable optical attenuator (VOA) that can arbitrarily attenuate the intensity of light to be output from the output port, by setting the angle of the reflecting mirror 141 shifted from an optimum coupled state. Specifically, as illustrated in FIG. 17, the angle of the reflecting mirror 141 is changed in the Y direction (ports array direction) to thereby change the position of the light which reaches the micro lenses 112 arranged at the end of the output port. Since the angle of the light entering in the end face of the optical fiber 111 through the micro lens 112 changes as illustrated in an enlarged view in FIG. 18 due to the change of the light reaching position, coupling efficiency of the light to the optical fiber 111 changes and the intensity of the light to be output from the output port Pout finally changes.
FIG. 19 is one example in which the angle of the reflecting mirror 141 with the largest coupling efficiency of the light to the optical fiber 111 at a certain output port Pout is designated as 0°, an angle change θ of the reflecting mirror 141 from 0° is plotted on the X axis, and a change in the intensity of the output light is plotted on the Y axis. Because the change in the intensity of the output light corresponds to a change of the coupling efficiency of the light to the optical fiber 111, the graph in FIG. 19 expresses a coupling efficiency function of the output port. It is seen from this graph that as the absolute value of the angle change θ of the reflecting mirror 141 increases, the coupling efficiency decreases.
Such a coupling efficiency function is a function that is mainly determined by the size and the shape of optical beam on the micro lens 112, a focal length of the micro lens 112, a focal length of a lens used for the light collecting optical system 130, a distance between the optical fiber 111 and the micro lens 112, a distance between the micro lens 112 and the light collecting optical system 130, and a distance between the light collecting optical system 130 and the reflecting mirror 141. The coupling efficiency function can also be obtained by theoretical calculation in a simple optical system. However, when the optical system becomes complex and the coupling efficiency function is to be derived accurately including influences of aberration of the lens system and the like, an optical design simulation is generally used.
In the above example, a case in which the angle of the reflecting mirror 141 is changed in the Y direction (ports array direction) has been described. However, the VOA function can also be realized by changing the angle of the reflecting mirror 141 in the X direction (angle distribution direction of respective wavelength lights), for example, as illustrated in FIG. 20. However, when the VOA function is realized by changing the angle of the reflecting mirror 141 in the X direction, it is known that there is the following problem relating to the wavelength (frequency) dependency of the intensity of the output light (for example, refer to Japanese Laid-open Patent Publication Nos. 2006-126678 and 2006-184472).
When the VOA function is realized in the wavelength selecting switch, the output intensity of a plurality of lights having different wavelengths can be attenuated separately for each wavelength. Therefore the transmission band characteristic expressing the wavelength dependency of the intensity of the output light becomes one important characteristic. The graphs illustrated in FIG. 21 and FIG. 22 are examples expressing the transmission band characteristic for a conventional wavelength selecting switch realizing the VOA function, in which a parameter where the frequency is normalized based on an interval between channels of the WDM light adjacent to each other is plotted on the X axis, and a change in the intensity of the output light of one channel is plotted on the Y axis. FIG. 21 illustrates a case in which the angle of the reflecting mirror is changed in the Y direction (ports array direction), and FIG. 22 illustrates a case in which the angle of the reflecting mirror is changed in the X direction (angle distribution direction of respective wavelength lights).
As illustrated in FIG. 21, when the VOA function is realized by changing the angle of the reflecting mirror 141 in the Y direction, even if the intensity of the output light is attenuated by shifting the angle of the reflecting mirror 141 from the optimum coupled state, the transmission band characteristic (graphs formed by round dots and triangular dots in the figure) at the time of attenuation becomes a trapezoidal shape the same as that of the transmission band characteristic (graph formed by squares in the figure) at the time of the optimum coupling. On the other hand, as illustrated in FIG. 22, when the VOA function is realized by changing the angle of the reflecting mirror in the X direction, this gives a characteristic where as the intensity of the output light is attenuated, protruding bumps appear at opposite ends of the transmission band.
Such protruding bumps in the transmission band characteristic are caused by a diffraction phenomenon where the optical beams incident on the reflecting mirror 141 enter into edges of the reflecting mirror 141 in the X direction and are rejected. If such protruding bumps occur, then when an optical amplifier is arranged on a subsequent stage of the wavelength selecting switch, optical components corresponding to the protruding bump portions are also amplified together with other optical components, thereby deteriorating the S/N ratio of respective channels. Accordingly, when the wavelength selecting switch is used to realize the VOA function, a desired control method is one where the angle of the reflecting mirror 141 is changed in the ports array direction.
However, even when the VOA function in the wavelength selecting switch is realized in the above manner by changing the angle of the reflecting mirror 141 in the ports array direction, there are two problems described below. A first problem is that the coupling efficiency of the light to the optical fiber 111 on the output side changes at the time of changing the angle of the reflecting mirror 141. In the reflecting mirror 141 using an MEMS mirror or the like, even if it is attempted to fix and control the reflecting surface at a desired angle, a certain angle change occurs due to external vibrations and changes in ambient temperature. If such an angle change of the reflecting mirror 141 occurs, the position of the light reaching the micro lens 112 at the output port also changes. Therefore, the coupling efficiency of the light to the optical fiber 111 on the output side also changes.
At this time, as illustrated in FIG. 23, a change δ in the intensity of the output light (coupling efficiency) accompanying an angle change b of the reflecting mirror 141 becomes noticeable at the time of setting attenuation (θa), as compared to at the time of setting optimum coupling (θ0). Moreover, as illustrated in FIG. 24, when an inclination of the coupling efficiency function is gradual, even if there is the angle change b of the reflecting mirror as described above, the change δ in the coupling efficiency decreases as compared to the case illustrated in FIG. 23. That is to say, from a standpoint of the change in the coupling efficiency of the light to the optical fiber 111 on the output side at the time of angle change of the reflecting mirror 141, the case where the inclination of the coupling efficiency function is relatively gradual is more advantageous.
A second problem is cross talk to an adjacent output port. In the wavelength selecting switch, the space in the ports array direction is limited due to restrictions on size and the like, and intervals between respective ports may be limited. Here, referring to a port of an output destination of the light as a signal port, and a port adjacent to the signal port as an adjacent port, if an interval between the signal port and the adjacent port is narrow, a part of the light guided to the signal port also leaks to the adjacent port, thereby causing cross talk.
FIG. 25 and FIG. 26 explain the cross talk to an adjacent port by using the coupling efficiency function. In the example of FIG. 25, when a case is considered where attenuation of the angle of the reflecting mirror 141 is set so that the intensity of the output light at the signal port becomes −15 dB with respect to that at the time of setting the optimum coupling, then as illustrated by the thick arrow line in the figure, the light leaks to the adjacent port by about −32 dB. Moreover the example of FIG. 26 illustrates a case where the inclination of the coupling efficiency function is sharper than in the case of FIG. 25, and in this case, even if the attenuation is set to −15 dB as above, and the port interval is the same, the amount of cross talk to the adjacent port becomes −50 dB or below. That is, the case where the inclination of the coupling efficiency function is sharp is advantageous from the standpoint of cross talk to the adjacent port.
As described above, in the conventional wavelength selecting switch, the two problems of the change in coupling efficiency of light to the fiber 111 on the output side at the time of the angle change of the reflecting mirror 141, and the occurrence of cross talk to the adjacent port, have a tradeoff relation with respect to sharpness of the inclination of the coupling efficiency function. Therefore, there is a problem that these cannot be reduced simultaneously.